A measuring as precise as possible of the mixing temperature at a certain time of the mixing process is of special importance for the control of the mixing process and the quality of the final product, for instance when rubber mixtures are compounded in rubber kneaders.
In this regard, it is known to let a temperature sensor project into the mixing chamber at certain places. Such temperature sensors may be provided in the form of a thermocouple or a resistance thermometer. As a result of the high stress during the mixing process the housing of such a sensor must be made very stable. On the other hand, an adequately stable design of the housing will result in considerable errors due to the thermal conductivity of the housing. Measuring errors of up to 15K may be caused.
Any attempts to reduce these measuring errors have so far essentially consisted in structural measures taken in the mechanical field. For instance, attempts have been made to achieve an improvement by means of a ceramic jacket and defined slots. However, the measuring error still amounts to 5K even with temperature sensors structured in such a way.
A further problem of temperature measuring with conventional sensors results from the fact that the thermal conductivity of the mixture continuously changes during the mixing process and that the coefficient of heat transmission between the mixture and the temperature sensor changes. On the one hand, this is due to the fact that the whole mixture usually consists of about 20 components gradually supplied to the mixing chamber according to certain criteria, while the mixing chamber always remains only partially filled. On the other hand, some mixture components change in condition during the mixing process. Natural caoutchouc and other elastomers, for example, change their molecular weight and their temperature. Carbon black and other components of many mixtures are added only in agglomerated form. These agglomerates are broken open and the particles mix with the elastomeres.
It is also known to detect the temperature of the mix in non-contacting manner by means of an infra-red sensor in the case of plastic mixtures, to which effect a window is provided in the mixing chamber and the infra-red sensor is to detect the temperature of the mix through this window. However, when applied this method has the disadvantage that the window pane gets dirty and scratched, so that further sources of error are added to the basic measuring errors caused by the measuring from outside through the pane.